When you say you added your sister to your existing reservation, do you mean the resort reservation or your
MDE reservation? If you just added her to the resort res, then it's understandable that you didn't get tags for her from DME - it's not an automatic thing. If you specifically made a separate DME reservation for your sister, that's a different situation.
In either case, the solution is the same - call DME directly at 866 599-0951. Do not call Disney reservations as they can't really help you other than to fill out a form and send it to DME. DME is a separate company, not owned or operated by Disney (Disney just contracts with them for the service). Calling DME is a quick way to solve the issue. Have your resort reservation number handy.
Steve